Re: [RFC PATCH 1/5] drivers : introduce device_path api

[Andy Green <andy.green@linaro.org>]

On 11/28/2012 12:37 AM, the mail apparently from Alan Stern included:
> On Tue, 27 Nov 2012, Andy Green wrote:
>
>>> I don't know if such an approach would be sufficiently general for
>>> future requirements, but it would solve the problem at hand.
>>
>> We can get a notification about device creation and do things there.
>> But the cost of that is like the tuple suggestion, they'll only be able
>> to do a fixed thing like operate on regulators.
>
> I'm not quite sure what you mean by that.  _Any_ function is capable of
> doing only a fixed thing.
>
>> Actually the targeted device may have arbitrary associated assets like
>> generic GPIO to turn on a "flash" for built-in webcam controlled
>> out-of-band.  These also can be reached by known names.  And the driver
>> may wish to do things with them that are more complex than enable /
>> disable or follow logical lifecycle of the hub or whatever.
>
> Let's worry about that when it arises.
>
>> However when the guidance from Greg is that we can do nothing except
>> complain at hardware designers for some reason I failed to quite
>> identify, I fear we are moving deckchairs on the titanic.
>
> Greg's advice was simply not to rely on pathnames in sysfs because they
> aren't fixed in stone.  That leaves plenty of other ways to approach
> this problem.

It's sage advice, but there is zero code provided in my patches that 
"relies on pathnames in sysfs".

> Basically, what you want is for something related to device A (the
> regulator or the GPIO) to happen whenever device B (the ehci-omap.0
> platform device) is bound to a driver.  The most straightforward way to
> arrange this is for A's driver to have a callback that is invoked
> whenever B is bound or unbound.  The most straightforward way to
> arrange _that_ is to allow each platform_device to have a list of
> callbacks.

Sorry I didn't really understand this proposal yet.  You want "A", the 
regulator, driver to grow a callback function that gets called when the 
targeted platform_device ("B", ehci-omap.0) probe happens.  Could you 
expand what the callback prototype or new members in the struct might 
look like?  It's your tuple thing or we pass it an opaque pointer that 
is the struct regulator * or suchlike?

Throwing out the path stuff and limiting this to platform_device means 
you cannot bind to dynamically created objects like hub or anything 
downstream of a hub.  So Felipe's identification of the hub as the 
happening place to do this is out of luck.

-Andy

-- 
Andy Green | TI Landing Team Leader
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